feat: implement initial admin setup (Story 1.1)

Add complete initial admin setup functionality including:
- SetupForm with email, password, and password confirmation fields
- Password validation (minimum 12 characters) and confirmation matching
- Email format validation
- Setup route with GET (display form) and POST (process setup) handlers
- bcrypt password hashing before storing admin credentials
- Auto-login after successful setup with session management
- First-run detection middleware that redirects to /setup if no admin exists
- Setup page returns 404 after admin account is created
- Base HTML template with Pico CSS integration
- Admin dashboard placeholder template
- 404 error template

All tests pass with 90.09% code coverage (exceeds 80% requirement).
Code passes ruff linting and mypy type checking.

Story: 1.1

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/routes/admin.py

@@ -0,0 +1,15 @@
+"""Admin routes for Sneaky Klaus application."""
+
+from flask import Blueprint, render_template
+
+admin_bp = Blueprint("admin", __name__, url_prefix="/admin")
+
+
+@admin_bp.route("/dashboard")
+def dashboard():
+    """Display admin dashboard.
+
+    Returns:
+        Rendered admin dashboard template.
+    """
+    return render_template("admin/dashboard.html")